This invention relates generally to microstrip antennas and microstrip antenna arrays and is particularly directed to microstrip antennas and arrays which are frequency-agile over a relatively large spectrum of frequencies. The invention also provides polarization diversity in frequency-agile microstrip antennas and arrays. This invention is also particularly directed to microstrip antenna arrays that are frequency-agile, have polarization diversity and can be electronically scanned. The frequency-agility is achieved without changing the physical dimensions of the antenna elements.
The microstrip antenna has been shown to be an excellent radiator for many applications requiring thin, inexpensive, conformal antennas which are rugged and have a low aerodynamic profile. However, their use has been limited by their inherent narrow operating bandwidth. A typical thin microstrip antenna will only operate over a frequency range of two or three percent. If it was desired to operate at some other frequency it was necessary to physically alter the dimensions of the radiating patch or to provide alternative radiating patches of different dimensions. Altering the dimensions was done by altering the length of the patch, by removing some of the conductor from the center of the patch or by placing small tabs of conductor along the edges of the patch. However, these alterations are permanent and do not allow for very rapid frequency changes. Another method of changing the operating frequency, which is not permanent, is to place electronically variable reactances along the edge of the conducting patch and connect them to the ground plane. These reactances, such as varactor diodes, require expensive, bulky, difficult to control equipment to provide precise analog bias voltages. If serveral antennas were required to track together in frequency, it was necessary to select matched varactors for each antenna or design custom bias networks for each antenna.
For many applications it is often highly desirable, especially when dealing with projectiles, missiles, aircraft and radar, to have antennas that have the capability of radiating energy over a wide range of frequencies with the frequency changes being made very rapidly. Since the instantaneous bandwidth of the microstrip antenna is small, i.e., approximately 2-3 percent, the capability of switching frequencies over a wide range would provide a considerable immunity to interfering signals. The prior art microstrip antennas do not have the capability of being switched rapidly and simply over a broad range of frequencies.
It is also highly desirable to hwave an antenna that has selectable polarization. To obtain polarization diversity in most prior art antenna it is necessary to have at least two antennas and associated power dividers, phase shifters and rf switches to provide complete polarization coverage. For mwany applications it would be beneficial to obtain polarization diversity with simple inexpensive equipment that is easily controlled or that can be controlled by a digital computer.
Another highly desirable feature for many applications is the ability to electronically scan the output beam of an antenna in conventional microstrip arrays. Because of the narrow range of the frequency response in prior art antenna arrays, it is necessary to either physically steer the array itself or to incorporate variable phase shifters into the feed lines of the individual antenna elements. These variable phase shifters are generally bulky, expensive and difficult to control.
This invention provides a method to achieve frequency-agility and polarization diversity in both individual antenna elements and arrays and a method to achieve electronic scanning in an antenna array. The described method is inexpensive, easily constructed and easily controllable.
It is therefore one object of this invention to provide a microstrip antenna which is capable of selectably radiating a wide spectrum of frequencies.
It is another object of this invention to provide a microstrip antenna that is capable of providing selectable polarization.
It is a further object of this invention to provide a microstrip antenna that is capable of being electronically scanned simply and easily by digital controls.
It is still another object of this invention to provide a microstrip antenna that provides selectable frequencies, selectable polarization and electronic scanning by means of simple electronic switching capable of being computer controlled, and thus instantaneously changable.
It is still a further object of this invention to provide a microstrip antenna constructed by standard printed circuit techniques that is conformal, has low profile and desirable aerodynamic qualities.